The present invention relates to a propulsion arrangement for a marine vessel, which propulsion arrangement comprises an internal combustion engine, a shaft arrangement and a steerable thruster unit, whereby the marine vessel further comprises an electric generator or a combined electric generator and motor in connection with the propulsion arrangement.
Conventional marine vessels with the above type of propulsion arrangements and thereto related machinery may be provided with an additional device, particularly an electric generator or a combined electric generator and motor for generating e.g. hotel load electric power or for improving flexibility and redundancy in propulsion power. Such devices are often called shaft generators which, depending upon their location, may be used only as an electric generator or as an electric generator and an electric motor in combination.
Basically, an electric generator may be connected to the free end of the internal combustion engine, i.e. opposite the main shaft of the internal combustion engine, whereby no extra gear is necessary. This solution requires surplus space at the free end of the engine. Furthermore, such a generator can not provide power through the engine, i.e. it can not function as an electric motor. This type of single function arrangement is usually called a PTO (Power Take Off) arrangement.
Alternatively, a combined electric generator and motor may be connected to the main shaft of the internal combustion engine by means of a so-called step-up gear. Naturally the step-up gear necessitates additional equipment with thereto related costs. The step-up gear also causes transmission losses. Furthermore, this known solution requires sufficient and suitable space along the main shaft. This type of dual function arrangement is usually called a PTO/PTI (Power Take Off/Power Take In) arrangement.
However, in view of the general concepts discussed above, the additional device should be clearly understood as having three separate functions, i.e. firstly the additional device may installed to be used as an electric generator (PTO), secondly to be used mainly as an electric motor (PTI), and thirdly to be used as a combined electric generator and motor (PTO/PTI), whereby it may coupled so that it in a first mode functions as an electric generator and in a second mode as an electric motor.
The object of the present invention is to achieve a propulsion arrangement for a marine vessel by which the above disadvantages are avoided and which provides a multifunctional operation by way of a simplified arrangement.